1. Field of the Invention
This invention pertains generally to resistor networks for terminating transmission lines and electronic devices and more specifically to a resistor network that is formed on an organic substrate.
2. Description of the Related Art
Transmission lines are used in a diverse array of electronic equipment to accommodate transmission of electrical or electronic signals. These signals may have a diverse set of characteristics, which might, for example, include direct or alternating currents, analog or digitally encoded content, and modulation of any of a diverse variety of types. Regardless of the characteristics of the signal, an ideal transmission line will conduct the signal from source to destination without altering or distorting the signal. Distance is inconsequential to this ideal transmission line, other than delays which might be characteristic of the transmission medium and the distance to be traversed.
At low frequencies and with direct current transmissions, many transmission lines perform as though they are nearly ideal, even over very great distances. Unfortunately, as the frequency of the signal increases, or as the frequency of component signals that act as a composite increases, the characteristics of most common transmission lines decay and signal transmission progressively worsens. This is particularly true when signals reach the radio frequency range or when transmission lines become longer. One common phenomenon associated with high frequency, long distance transmission lines is a loss of the signal's high frequency components and the introduction of extraneously induced interfering high frequency signals. Another common phenomenon is echo or line resonance, where a signal may be reflected from one end of the transmission line back to the other. In the case of a digital pulse, the effect will lead to corrupted data, since additional pulses may be received that were not part of the original transmission, and reflected pulses may cancel subsequent pulses.
To prevent echo within a transmission line, it is possible to terminate the line with a device which is referred to in the art as an energy dissipating termination. The termination must have an impedance which is designed to match the characteristic impedance of the transmission line as closely as possible over as many frequencies of interest as possible. Transmission lines generally have an impedance which is based upon the inductance of the conductor wire, capacitance with other signal lines and ground planes or grounding shields, and resistance intrinsic in the wire. With an appropriate transmission line, the sum of the individual impedance components is constant and described as the “characteristic impedance.” To match the transmission line characteristic impedance over a wide frequency range, a termination must also address each of the individual impedance components. The effect of inductance is to increase impedance with increasing frequency, while capacitance decreases impedance with increasing frequency. Intrinsic resistance is independent of frequency.
In the particular field of data processing, transmission lines typically take the form of busses, which are large numbers of parallel transmission lines along which data may be transmitted. For example, an eight bit data bus will contain at least eight signal transmission lines that interconnect various components within the data processing unit. The data bus is actually a transmission line having to accommodate, with today's processor speeds, frequencies which are in the upper radio frequency band approaching microwave frequencies. These high frequency busses are, in particular, very susceptible to inappropriate termination and transmission line echo.
Terminations used for these more specific applications such as the data processor bus serve several purposes. A first purpose is to reduce echoes on the bus by resistively dissipating any signals transmitted along the bus. This first purpose is found in essentially all termination applications. A second purpose, more specific to data busses or other similar electronic circuitry, is to function as what is referred to in the art as a “pull-up” or “pull-down” resistor. The termination resistor will frequently be connected directly to either a positive power supply line or positive power supply plane, in which case the termination resistor is a “pull-up” resistor, or the resistor may be connected to either a negative or ground line or plane, in which case the resistor is referred to as a “pull-down” resistor. When no signal is present on the line, the voltage on the transmission line will be determined by the connection of the termination resistor to either a power supply line or a ground or common line. Circuit designers can then work from this predetermined bus voltage to design faster, more power-efficient components and circuits.
The prior art has attempted to address signal line termination in a number of ways which were suitable at lower operating speeds and frequencies, but which have not proven fully desirable as frequencies and components thereof increase.
One of several processes may be used to fabricate resistors. One such process is referred to as thin film, which might include vapor deposition techniques, sputtering, semiconductor wafer type processing, and other similar techniques. An example of a thin film component is found in U.S. Pat. No. 5,216,404 to Nagai et al.
Another process is to use thick film components, herein considered to be components that are formed from a layer of semi-conductive metal oxide, cermet or a dielectric material deposited upon a non-conductive substrate, are most commonly formed from screen printing techniques. For the purposes of this application, thick films are defined as films formed when specially formulated pastes or inks are applied and fired or sintered onto a substrate at a high temperature of around 900 degrees Centigrade in a definite pattern and sequence to produce a set of individual components, such as resistors and capacitors, or a complete functional circuit. The substrates can be either pre-fired or can be in a green un-fired state. The pastes are usually applied using a screen printing method and may typically have a thickness of from 0.2 to 1 mil or more, and are well known in the industry. Cermet materials are materials comprising ceramic or glass in combination with metal compositions, where the first three letters: CER & MET make the word CERMET.
TCR stands for Temperature Coefficient of Resistance, which is a measure of the amount of change in resistance over some temperature range. Sheet resistance for the purposes of this disclosure is measured in the units of ohms per square. This will be considered herein to be the resistance of a film of equal length and width.
Low TCR thick film resistors may be readily manufactured that are both durable and have excellent TCR. These resistors may have sheet resistances that vary from fractions of an Ohm to millions of Ohms per square with a TCR less than ±100 ppm/□C. Inductance increases with length. Therefore, to minimize inductance in the termination, signal lines should be kept as short as possible. Furthermore, shorter line lengths decrease the undesirable cross-talk described hereinabove. Stray capacitance should be minimized, since this stray capacitance is frequently variable with temperature due to temperature related variations in ordinary dielectrics.
In the prior art, transmission line terminations were initially constructed using large Cermet resistors which were formed by thick film techniques upon alumina (aluminum oxide) substrates. These components were then mounted into a circuit board in a Single-In-line Package (SIP) format. Later, Ball Grid Array (BGA) packages were developed for integrated circuit packages. In this package, the connection between a printed circuit board and the BGA component is achieved through the use of a number of solder balls. These balls are not limited to placement around the periphery of the device, as was the case in the chip resistors of the prior art, but instead the BGA has terminations distributed in the array across the entire package resulting in a higher packaging density. Examples of BGA type terminations are found in U.S. Pat. No. 4,332,341 to Minetti; U.S. Pat. No. 4,945,399 to Brown et al; U.S. Pat. No. 5,539,186 to Abrami et al; U.S. Pat. No. 5,557,502 to Banerjee et al; U.S. Pat. No. 5,661,450 to Davidson; U.S. Pat. No. 6,097,277 to Ginn; U.S. Pat. No. 6,326,677 to Bloom; U.S. Pat. No. 6,005,777 to Bloom; U.S. Pat. No. 5,977,863 to Bloom; U.S. Pat. No. 6,246,312 to Poole and U.S. Pat. No. 6,194,979 to Bloom. Each of these patents illustrate various types of BGA components and packages, the contents and teachings which are incorporated herein by reference.
These prior art BGA devices have a high cost of production due to the fact that they are manufactured either one at a time or in small arrays and many manufacturing steps are required.
While the prior art has provided devices suitable as resistor networks, there is a current unmet need for a lower cost resistor network with low inductance that can be fabricated using an efficient manufacturing process.